Former Gov. Tim Pawlenty on Thursday released the first TV ad of his comeback bid for governor, a sharply negative attack on his Republican primary opponent, Jeff Johnson.

“Who is Jeff Johnson? He’s a career politician who taxes and spends,” says the ad, citing votes and comments on sales and property taxes and the Affordable Care Act.

Johnson, a Hennepin County commissioner, counter­punched at a Thursday news conference. The sparring is a contrast with the three-way DFL primary for governor, where the candidates have refrained from public attacks against one another in recent weeks.

“The day that he attacks is the day he knows he might lose. We got there earlier than I thought we would,” Johnson said. He also said it was “possibly the most dishonest ad I have ever seen.”

Johnson said the claim about him proposing a property tax increase refers to a 2009 matter before the Hennepin County Board. After fellow board members proposed a 4.98 percent property tax increase, Johnson said he pitched a lower tax increase of 3 percent in an attempt to goad his colleagues into a more fiscally conservative position.

As for the charge that Johnson pushed for a sales tax expansion, the Pawlenty ad refers to an opinion piece Johnson wrote in which he agreed with the principle of broadening the sales tax base while lowering the rate. But in the piece, Johnson actually opposed a proposal from Gov. Mark Dayton that was being debated at the time.

Johnson said if a member of his staff proposed an equivalent ad about Pawlenty, Johnson would fire the person for dishonesty.

Pawlenty, who served as governor from 2003 to 2011, had previously declined to engage with Johnson and has been seen as the front-runner to win the Aug. 14 GOP primary. Johnson was the GOP candidate for governor in 2014, losing to Dayton, and he again won the Republican endorsement at the party’s state convention in June.

Pawlenty’s ad, which started airing on Thursday, shows him taking no chances in an unstable political environment against the possibility that Johnson is gaining ground with conservative voters.

J. Patrick Coolican is a Star Tribune political reporter who began his career by starting a website and following the 2000 presidential campaign. He's been a journalist in seven different states, and in 2013-14 was a Knight Wallace Fellow at the University of Michigan.

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